Optical performance monitoring of power, wavelength, and optical signal-to-noise ratio is required in wavelength-division-multiplexed (WDM) networks for providing quality of service and assuring network survivability. While amplified spontaneous emission (ASE) is unpolarized, the data signal remains almost polarized during transmission. Hence polarization extinction (PE) has been employed to determine the OSNR after measuring the ASE level. In the approach based on the polarization extinction, monitoring of the OSNR was demonstrated by measuring the polarization extinction ratio of WDM signals.
FIG. 1 shows two proper measurement setups based on the polarization extinction. They utilize a polarization controller 101 including a polarizer in front of either a conventional optical spectrum analyzer (OSA) 103 or a tunable optical filter 105 followed by a power meter 107. First the state of polarization of the incoming WDM signal is changed by means of the polarization controller 101 until the optical spectrum analyzer 103 or the power meter 107 indicates minimum power in the channel under investigation. For ASE, the reading will show half the ASE power. Then the polarization controller 101 is set to the orthogonal state. The displayed power is a maximum and corresponds, in case of sufficiently high OSNR, to the signal power. With these values, the OSNR is calculated.
The significant feature of the above PE technique is that this approach requires only a low-cost optical spectrum analyzer. However, a minute change in outside temperature could affect the polarization state of DWDM signals in transmission fiber. Thus, for the use in a practical network, it is necessary to adjust the polarization state of each channel. A simple technique that can be used to monitor the OSNR automatically is using the polarization-nulling method. This technique, based on the polarization-nulling method, was implemented by using a rotating quarter-wave plate and a rotating linear polarizer.
FIG. 2 shows the experimental setup to demonstrate the operating principle of the polarization-nulling technique. In the experimental setup, it uses a tunable laser 201 and an ASE source 203 to simulate an optical signal with various OSNR. Thus, the OSNR could be changed easily by adjusting the variable attenuators placed in front of these optical sources The optical signal was filtered by 1*8 waveguide grating router (WGR) 205, and then sent to the OSNR monitoring module 207 via an optical attenuator. The state of polarization of the optical signal incident on the OSNR monitoring module could be linear, circular, or elliptical. However, this arbitrarily polarized signal could be changed to a linearly polarized signal simply by using a rotating quarter-wave plate. The linear polarizer, placed at the output of the quarter-wave plate, was also rotated slowly.
Thus, the signal power (together with the polarized ASE noise) was measured whenever the linearly polarized signal from the rotating quarter-wave plate was aligned to the rotating linear polarizer. The polarized ASE could be measured when the linear polarizer was in the orthogonal state with the linearly polarized signal obtained by the rotating quarter-wave plate. The OSNR therefore could be monitored by comparing the maximum and minimum values of the detected signals.
U.S. Pat. No. 5,223,705 discloses a measurement of an optical amplifier parameter with polarization by using a polarization controller and a linear polarizer for measuring the ASE noise and gain of an optical amplifier. By sequentially adjusting the polarization controller, sequential measurement from the linear polarizer can have the sequential output of minimum and maximum optical powers. The minimum optical power represents the half ASE value, while the maximum optical power represents sum of the half ASE and optical signal values. Therefore, this approach can measure the ASE noise and gain of an optical amplifier.
The above mentioned techniques utilize a combination of a polarization rotator and a linear polarizer to measure the OSNR. The measurement is based on the assumption that the signal is polarized while the noise is not. To improve the sensitivity of monitoring and enable multi-parameter monitoring, a compact module for simultaneous channel and OSNR monitoring is provided in the present invention.